At 10:00
AM, Friday, October 20, 2000, Barry Barish, Director of the LIGO Laboratory
and Linde Professor of Physics at Caltech, along with Dr. Stan Whitcomb,
leader of the LIGO Commissioning effort, announced that "First Lock"
had been achieved with the two-kilometer long interferometer at the LIGO
Hanford Observatory. This marks the fulfillment of a major LIGO milestone,
the first time laser light has resonated throughout a full LIGO detector.
Additionally, all mirrors were "locked" into their proper positions
to atomic-scale precision using a sophisticated computer-based control
system. First lock validates many aspects of the control system design
for the initial LIGO detectors, but it has even greater significance as
the beginning of the process of tuning the interferometer to its full
sensitivity. Most importantly, as Barish points out, "This achievement
brings us closer to our real goal--LIGO's first gravitational-wave observations."

Beam Splitter
Chamber for 2K Interferometer

The Washington
two-kilometer interferometer, WA2K as it is called, has long optical cavities
that span the two kilometer distance from a "mid-station" on
each arm of the observatory to the corner station building, which also
houses the laser, beam splitter, recycling mirror and photodetectors.
Light incident on the beam splitter is divided into beams that enter the
optical cavities in each arm. When the interferometer is locked into resonance,
this light bounces back and forth up to fifty times in each arm, exactly
reproducing its path and spatial pattern on each round trip. This causes
a large build up of stored light in the long cavities. Light leaking back
from the two arms toward the beam splitter will interfere and return toward
the laser because of the careful positioning of the mirrors by the control
system. Any imbalance between the two arms is immediately removed by adjusting
the positions of the beam splitter and cavity mirror. The adjustment forces
can be measured to indicate the presence of any gravitational-wave or
other perturbing force acting on the instrument. The recycling mirror
traps the light returning toward the laser and sends it back toward the
beam splitter, causing the light on the beam splitter to grow much more
powerful than the light emitted by the laser.

Interferometer
Locked!

Video cameras
monitor the build-up of light in the interferometer, which is displayed
live in the control room. In the photo at left, the top two images on
the screen show the light built up in the long cavities, while the image
on the lower left shows the built up recycled laser light at the beam
splitter. These images are dark in the absence of resonance, when only
the laser light shining on the recycling mirror (shown at lower right
on screen) can be seen. Also present in this photograph is Professor Rainer
Weiss of MIT (standing at center in front of screen), who in 1973 originally
proposed building a gravitational-wave detector using laser interferometry.
He has worked decades toward this accomplishment.

Scientists in
Triumphant Repose

Achieving
first lock required tight coordination between the many scientists and
engineers who contributed to the design, construction, installation and
commissioning of the detector. Stan Whitcomb, shown at right seated amongst
a triumphant group of scientists, directed the successful commissioning
effort. Despite the achievement, Stan is mindful of the work yet to be
done. "The detector control systems must be carefully characterized
and tuned to achieve maximum sensitivity and reliable operation. And,
of course, this is just the first of three interferometers that we have
to commission." Indeed scientists Matt Evans (in red shirt) and Nergis
Mavalvala (kneeling at console) remained focused on improvements to the
computer lock-acquisition codes even as this photo was taken.

Currently,
the WA2K interferometer is the largest precision optical device in the
world. Yet that title will soon be surpassed when the Louisiana four-kilometer
(LA4K) interferometer comes on line in Livingston. Then will follow Washington's
four-kilometer interferometer, currently being installed next to WA2K
at Hanford. In the near future, WA2K will be put through its paces during
a week-long second engineering run. Future commissioning efforts and engineering
runs are expected to lead to improvements in the sensitivity and reliability
of each of the interferometers, climaxing in the first LIGO science runs,
slated for 2002. A lot has been accomplished, and much still remains to
be accomplished. At the end of his talk on Friday, Stan put everything
into perspective by comparing "first lock's" significance for
LIGO to the significance the Wright brothers first flight had for aviation.
"It doesn't stay up that long. It isn't very far off the ground.
But it does fly!"